A subdomain is a part of a web address which is under the main domain, for example name.example.com. Technically, even in www.example.com the "www" element is a subdomain because the fully qualified domain name is only "example.com". Every single subdomain can have its own site and records and can also be hosted using a different provider if you need to use a certain feature that is not offered by your current service provider. An example for using a subdomain is if you have a company website along with an online store under a subdomain where clients can acquire your products. Additionally, you can have a forum where they can talk about the products and by employing subdomains as opposed to subfolders you'll avoid any chance of all websites going down if you perform maintenance, or update one of the site scripts. Keeping the sites separated is more secure in case of a script security breach.